The present invention is directed to methods of aligning liquid crystals in a liquid crystal display by applying low molecular weight photo-crosslinkable materials onto a substrate in a liquid crystal display, and then reacting the photo-crosslinkable materials.
Liquid crystal displays (LCDs), such as twisted-nematic liquid crystal displays (TN-LCDs) or other LCDs whose optical properties depend on the effect of an applied electric field, require the liquid crystal alignment to be controlled. The conventional method of achieving this control is by confining the liquid crystal layer between two mechanically rubbed surfaces. The liquid crystal molecules adjacent to a surface are aligned in the rubbing direction.
In addition to the drawbacks of generating dust and electrostatic charges, rubbing methods are not adequate to obtain multidomain alignment structure for high-resolution angle active matrix LCDs having the wide viewing angle and high contrast ratio.
These problems can be solved using the photoalignment technique reported in U.S. Pat. Nos. 5,389,698 and 5,838,407 to Chigrinov et al. In this method, anisotropic surfaces of the aligning layers are created using polarized light or a combination of polarized and non-polarized light irradiation. Cinnamate- and coumarin-containing polymers are usually used for this photoalignment technique because of their high photo- and thermal-stability of the induced alignment. Stable anisotropy in such materials is induced through the photo-dimerization (crosslinlking) of photosensitive units. However, this method teaches the use of polymers, preferably of high molecular weight that are applied to a substrate and then photo-irradiated.
Another method used in the art for aligning liquid crystals is reported in U.S. Pat. No. 6,001,277 to Ichimura et al. Ichimura ""277 uses a resin that contains a photoisomerizable and dichroic structural unit. Ichimura ""277, however, does not disclose a resin with a photo-crosslinkable group, or applying an unreacted material to the substrate and then reacting and aligning by photo-irradiation.
U.S. Pat. No. 5,568,294 to Lee discloses a liquid crystal display having a liquid crystalline polymer as an alignment layer that is disposed in the liquid crystal display proximal to the liquid crystals in the display. Lee ""294 adds a polymer layer disposed underneath the liquid crystalline polymer. Lee ""294, however, does not disclose an alignment layer with a resin containing a photo-crosslinkable group, or applying an unreacted material to the substrate and then reacting and aligning by photo-irradiation.
The art lacks a photoalignment method using low molecular weight photo-crosslinkable materials that are applied to a substrate and then irradiated with light to react the material and induce anisotropy.
It is an object of the invention to provide a method for aligning liquid crystals by applying low molecular weight photo-crosslinkable materials onto the substrate in a liquid crystal cell and then reacting the material and inducing anisotropy in the reacted material.
The present invention provides a method for forming an alignment layer for aligning liquid crystals comprising: dissolving a photo-crosslinkable material in a solvent, wherein said photo-crosslinkable material contains at least one polymerizable group and at least one photosensitive group, and wherein said photo-crosslinkable material has a weight average molecular weight less than about 1,000; depositing said photo-crosslinkable material and said solvent on a substrate; removing the solvent; and irradiating the photo-crosslinkable material selectively with linearly polarized ultra-violet light to provide an alignment layer.
The present invention also provides a method of forming a liquid crystal display comprising: providing two opposed substrates each covered with an electrode; dissolving a photo-crosslinkable material in a solvent, wherein said photo-crosslinkable material contains at least one polymerizable group and at least one photosensitive group, and wherein said photo-crosslinkable material has a weight average molecular weight less than about 1,000; depositing said photo-crosslinkable material and said solvent on at least one of the electrode covered substrates on the surface facing the other substrate; removing the solvent to form a liquid crystal film; irradiating the photo-crosslinkable material selectively with linearly polarized ultra-violet light to provide an alignment layer; placing spacers between the substrates; sealing three of the sides of the substrate to form a cell; filling the cell with a liquid crystal; and sealing the cell.
The present invention also provides a liquid crystal display comprising a first and second cell wall structure, electrodes disposed on facing sides of said first and second cell wall structures, an alignment layer disposed on at least one of said electrodes, and liquid crystals disposed within a space between the first and second cell wall structures, wherein the alignment layer comprises a photo-reaction product of a photo-crosslinkable material that contains at least one polymerizable group and at least one photosensitive group, wherein said photo-crosslinkable material has a weight average molecular weight less than about 1,000.